feat: add /update skill for pulling upstream changes (#372)

Interactive skill that guides Claude through fetching upstream NanoClaw,
previewing changes, merging with customizations, running migrations, and
verifying the result. Includes:

- SKILL.md with 9-step update flow
- fetch-upstream.sh: detects remote, fetches, extracts tracked paths
- run-migrations.ts: discovers and runs version-ordered migrations
- post-update.ts: clears backup after conflict resolution
- update-core.ts: adds --json and --preview-only flags
- BASE_INCLUDES moved to constants.ts as single source of truth
- 16 new tests covering fetch, migrations, and CLI flags

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

scripts/post-update.ts

@@ -0,0 +1,5 @@
+#!/usr/bin/env tsx
+import { clearBackup } from '../skills-engine/backup.js';
+
+clearBackup();
+console.log('Backup cleared.');